Process for forming a web of synthetic fibers



particles is deposited concomitantly from a. liquid suspension .upon a screen, to bond a sub- PRGCESS FOR FORMING A WEB F SYNTHETIC FIBERS Kennett Square, Pa., assiguor Wilming- Anton Fridrich Fridrichsen, to E. I. du Pont de Nemours and, Company,

ton, Del., a corporation of Delaware No Drawing. FiledDec. 17, 1959, Ser. No.

' SClaims. (Cl. 162-146) This invention relates to non-woven structures and more specifically to a process for treating non-woven structures comprising-synthetic staple fibrous mater1al.

Non-woven sheet materials from naturally occurring fibers are well known. They are widely used in the form of paper, wool felts and the like. With the development of synthetic fiber materials, there has been much study given to Ways in which synthetic fibers could be formed into non-woven sheetsand sheet-like structures to take advantage of the simple processing steps which are employed in the manufacture of non-woven fabrics from naturally occurring fibers while retaining at the same time the advantages of high strength, high abrasion resistance, and'indifierenceto water which characterize many of the synthetic fiber materials.

One of the main difficulties in the utilization of synthetic fibrous materials in non-woven structures has been due to poor self-bonding characteristics of such fibers. Thus in the past production of such structures required the use of thermoplastic resins as bonding materials, and solvent bonding techniques. More recently the use of fibrids for bonding synthetic fiber materials has been taught. Fibrids are a form ofsynthetic fibrous material described more fully in Belgian Patent 564,026.

The-term fibrid is employed herein to designate a non-rigid, wholly, synthetic polymeric particle capable of forming paper-like structures upon a paper-making machine. Thus to be designated a fibrid, a particle must possess (a) an ability to form a 'waterleaf having a couched wet tenacity of at least about 0.002 gram per denier when a plurality of the said particles is deposited from a liquid suspension upon a screen, which waterleaf, when dried a't a'temperature below about 50 C., has a dry tenacity at least equal to its couched wet tenacity and (b) an ability, when a plurality of the said with staple fibers stantial weight of the said fibers by physical entwinement of the said particles with the said fibers to give a composite waterle-af with a wet tenacity of at least about 0.002 gram per denier. In addition, fibrid particles have a Canadian freeness number between 90 and 790 and a high absorptive capacity for water, retaining at least 2.0 grams of water per gram of particle under a compression load of about 39 grams per square centimeter. Tests by which these measurements are made are detailed below.

Any normally solid wholly synthetic polymeric material may be employed in the production of fibrids. By normally solid is meant that the material is non-fluid under normal room conditions. By an ability to bond a substantial weight of... (staple) fibers is meant that at least 50% by weight of staple based on total staple and fibrids can be bonded from a concomitantly deposited mixture of staple and fibrids.

It is believed that the fibnid characteristics recited above are a result of the combination of the morphology and non-rigid properties of the particle. The morphology is such that the particle is non-granular and has at least 3,101,294 I'Patented Aug. 20, 1963 ICC fibrid particles are not'identical'in shape and may include both fiber-like and film-like structures. The nonrigid characteristic of the fibrid, which renders it extremely supple in liquid suspension and which permits the physical entwinement described above, is presumably due to the presence of the minor dimension. Expressing this dimension in terms of denier, as determined in accordance with the fiber coarseness test described in Tappi 41, 175A-7A, No. 6 (June 1958), fibrids have a denier no greater than about '15.

Complete dimensions and ranges of dimensions of such heterogeneons and odd-shaped structures are difiicultto express. Even screening classifications are not always completely satisfactory to. define limitations upon size since-at times the individual particles become entangled with one another or'wrap around the wire meshes of the screen and thereby fail to pass through the screen. Such behavior is encountered particularly in the case of fibrids made from soft (i.e., initial modulus below 0.9) polymers. As a general rule however, fibrid particles, when classified according to the Clark Classification Test (Tappi 33,

294-8, No. 6 (June 1950) are retained to the extentofnot over 10 on a Ill-mesh screen, and retained to the extent of at least 90% on a ZOO-mesh screen.

Eibrid particles are'usually frazzled, have a high specific surface area, and as indicated, a high absorptive capacity for water.

Preferred fibrids are those the waterleaves of which when dried for. a period of 12 hours at a temperature below the stick temperature of the polymer from which they are made (i.e., the minimum temperature at which a sample of the polymer leaves a wet molten trail as it is stroked with a moderate pressure across the smooth surface of a heated block) have a tenacity of at least 0 .005 per denier. I

By the use of fibrids it is possible to prepare nonwoven fabrics from synthetic fiber staple material and fibrid binding agents which have a high as-formed strength. However, for development of a maximum strength and many other properties it has been desirable,

. 'as the already-mentioned Belgian patent indicates, to

0 the range of 190 C. to 220 C. or higher. Suoh temperaone dimension of minor magnitude relative to its largest dimension, i.e., the fibrid particle is fiber-like or filmlike. Usually, in any mass of fibrids, the individual tures fuse the fibrids and under proper conditions only minor damage is done to the staple fibers.

It is an object of this invention to provide a simplified and improved procedure for bonding-of non-woven fibridbonded fibrous sheets from synthetic fiber materials and in particular to provide a means of bonding such sheets employing substantially lower temperatures whereby there is no damage to the staple fiber material and whereby there is accordingly obtained a stronger and more uniform sheet. Other objects will become apparent from the description which follows. These objects are achieved in a process which consists of warmpressing a wet composite sheet of synthetic fiber staple and fibrids at a temperature at least about 20 C. below the melting point of the lowest melting polymeric material in the sheet, and below about 160 C., and pref- I erably at a temperature between C. and C., the sheet containing, at the start of the Warm pressing treatment, at least about 10% by weight of water. As a result of such processing there is obtained a non-Woven sheet consisting essentially of fibrous materials; that is,

a the calendering process, employing a temperature below 160 C., does not fuse the binder material completely. In particular, while temperatures in the range of 140 C. to 160 C. are useful, it is particularly desirable to use temperatures below 140 C., since in this range the above indicated processing conditions result in spot-fusion of the fiber materials throughout the structure without destruc tive coalescence of the fibrous form of the material. There is thus provided a uniform bonded, fibrous sheet of highmel-ting synthetic polymer material in which the bonds are provided by fibrids heated sufiiciently to coalesce them without completely destroying their fibrous form.

The surprising feature of the present invention is that by the use of a modest-amount of water it is possible to obtain a uniformly and completely bonded material while employing mild temperatures below the melting point of both the staple fiber. material and the fibridscomprising the. sheet. s

The following examples illustrate the principles and practice involved in the present invention without indicating any limitations thereto.

Example 1 A solution is prepared by adding 15 parts of 20/80 6-6/ 6 nylon to 85 parts of N,N-dimethyl formamide and heating to 130 C. Sufficient zinc stearate is added so that it represents 20% of the total weight of solids. Fibrids are prepared from solution by adding it to a 70/30 water/-N,N-dimethylformamide mixture at 27 C. in a S-qt. Waring Blendor operating at approximately 7,000 rpm. The fine, uniform fibrids obtained are filtered and washed free of solvent.

A water slurry of nylon staple fiber, A" in length, 1.5 denier per filament is prepared by adding one part of the staple fiber to 10,000 parts of water. This slurry is agitated gently .and to this is added sufiicient fibrids to provide a slurry containing 30% of the fibrids prepared as described above. The slurry is agitated for several minutes and then is poured into a 10" by 20" sheet mold onto a screen and permitted to drain. The unpressed paper obtained therefrom is dried at 105 C. to a water content of less than in an oven. It is then calendered at 210 C. This treatment causes discoloration and subsequent degradation of paper properties. However, when the experiment is repeated using the same components but employing the process conditions of the present invention, much improvement in appearance of the sheet is noted. Specifically, the hand sheet after formation, containing 100% water by weight, is passed through the nip of a set of calender rolls at 115 C. with a nip pressure of 140 lbs/in. The speed of the sheet through the nip is approximately 30 ft./min. With this treatment the sheet is obtained in a dry for-m with an appearance much better than the sheet obtained in the oven drying technique described above and equal to or better than sheets processed by calendaring at 190 C. with 1500 lbs/in. or higher nip pressure. In particular, no discoloration was observed. Further, the warm wet calendering treatment gave a paper which did not adhere to either of the rolls of the calender.

Example 2 In another paper preparation following the technique of Example 1, fibrids prepared from a copolyamide of 720% poly(hexamethylene adipamide) and 80% polycapnoamide are employed in the preparation of a nylon paper using staple fiber of 66 nylon, 3 denier per filament, long. The slurry is prepared with 90% staple fibers and of the fibrid binder. Water is removed to a 50% water content from the sheet formed. The calender is operated at a temperature of 125 C. under a pressure of 140 pounds per inch and at a speed of 30 feet per minute. The sheet formed has a high tensile strength and no discoloration :or degradation of the polymer components.

Example 3 A synthetic fiber paper of polyamide fibrous materials is prepared containing 80% of A", 3 denier per filament, staple fibers of poly (hexamethylene adipamide) and 20% of the copolyamide paper binder described in Example 1. From this mixture a 1.9 oz./sq. yd. paper is obtained. This paper is found to have a moisture content of about 80% after wet pressing. Portions of the paper are then calendered in three different treatments at increasing temperatures from 110 C. to 125 C. A constant nip pressure of 200 lbs/in. is employed, and the speed is 10 ft./ min. At the three different temperatures, there is no noticeable difference in paper properties. V In each case the paper is still moist after calendering. The average tensile strength as determined on an Instr-on tester is 9.0 lb-s./ in./oz./s'q. yd. in the machine direction and 10.1 lbs./in./ oz./ sq. yrd. in the cross direction. Microscopic examina tion shows that the fibers are visually unhurt and that while the binders are fused, their fibrous identity is not d stroyed. Pressing of the dried sheet in a flattened press at 190 C. and 625 lbs/sq. in. gives a sheet with a slightly increased tensile strength, but the fibrids are completely fused in the process and lose their fibrous identity.

In a comparativecontrol the couched Wet sheet is dried at 105 C. to a moisture content below 5% and thereafter calender-ed at 115 C. at 200 lbs/in. and 10 ft./min. The average tensile strength in both directions is very low and estimated to be below about 2 libs./in./oz./ sq. yd. Microscopic examination discloses there is no fusion of the binder whatever.

Example 4 30 grams of a solution containing 10% by weight solids of ethylene terephthalate/ethylene isophthalate copolyrner (/20) in tri-fiuoroacetic acid is added in an even stream to a mixture of 350 ml. of glycerol and 50 ml. of water in a Waring Blender at approximately 14,000 rpm. The fibrids obtained are filtered and'washed with water until free of organic liquid.

In a similar experiment, a synthetic fiber paper is o tained from staple fibers consisting of Dacron polyester fiber, 3 denier per filament, A" length and fibrids prepared as described above. The aqueous slurry of fibrous materials contains 78% of the staple fibers and 22% of the fib-rids. From this slurry there is prepared a paper with a weight of 2.1 oz./sq. yd. This paper as formed is found to contain 60% by weight of water after couching and wet pressing. The sheet is calendered at C., under a pressure of 250 lbs/in. and at a speed of 10 tL/min, giving a sheet with a tensile strength of 8.4 lbs./in./oz./ sq. yd. in the machine direction and 10.2 lbs./in./oz./ sq. yd. in the cross direction. As before, there is only partial fusion of the fibrid materials resulting in a sheet with substantially a completely fibrous content.

The nature of the filbre employed in the sheet products of the-present invention is not critical. It may be a natural fibre such as wood pulp, cotton, flax, wool or the like or a fibre prepared from a synthetic or manmade fiber may be used provided it is non-fusible at a temperature below about C. Preferably the fibers comprising the sheet are prepared from polymers having a melting point above about 200 C. and a moisture regain of above about 1%. In a similar manner the chemical identity of the fibrid is not critical in the process of the present invention. The fibrids may be prepared from any linear synthetic polymer. Preferably they are obtained from a polymer which is water insoluble, has a melting point above about 180 C. and a second order transition temperature above about 0 C.

The amount of pressure employed in the calendering operation will vary with the thickness of the waterleaf and the temperature employed in the warm pressing operation. In general the use of higher pressures permits the operation to lbe perfiormed at lower temperatures as will be apparent to those skilled in the art. The processing of a sheet of high basis weight will in general require the use of a higher pressure and adjustment of calendering speed to permit thorough and even distribution of heat. Pressures as low as lbs. per inch on the calender roll may be used for appropriate fibrids, particularly where the basis weight of the sheet is low. The use of pressures as high as 600 lbs. per inch is not unusual. Where static pressures are used a range of from about lbs/sq. in. to about 12 00 lbs/sq. in. in the pressing operation is recommended.

It is readily apparent from the foregoing that the present invention provides a simplified and economical process for the preparation of strong papers from synthetic fiber materials. It is already known that fibrids offer superior bonding properties and that their use perunits the preparation of unfused and unbonded papers of adequate strength. However, for critical uses, it has heretofore been considered necessary even with fibrid binders, to heat synthetic fiber papers at a temperature sutficiently high to completely melt or fuse the fibrid binder particles. Now, however, by the practice of the present invention it is possible to prepare a synthetic fiber paper containing; predominantly fibrous materials even after bonding. In the preferred range of temperatures, the process herein described provides papers which are equivalent in strength to completely fused papers but which still contain the fibrid-binding agent in a fibrous form. Moreover, the bonding 'calendering treatment required for the production of such materials makes much less stringent demands upon the machinery involved. Temperatures in the range preferred for the present invention, that is, 105 C. to 140 C., are much easier to obtain and much easier to control than the highertemperatures heretofore considered necessary. -In addition, because of the low temperatures involved, there is no chance that the full strength of the staple fiber materials employed in the sheet can be impaired in any way. Thus, by the practice of the present invention superior sheets are obtained in a process which oifers substantial advantages and improvements over those hitherto considered necessary.

Many equivalent modifications will be apparent to those skilled in the art from a reading of the above without a 'departure from the inventive concept. I

What is claimed is:

1. In the process for forming a non-woven, tusion bonded, composite sheet from synthetic polymer staple fiber and fibrid binding agent, saidpolymer being selected from the group consisting of poly(hexamethylene adipamide), polycaproamide, polyethylene terephthalate,

copoiymers containing hexamethylene adipamide and caproamide units, and copolymers containing ethylene terephthalate and ethylene isophthalate units, by concomitantly depositing an aqueous slurry of a mixture of said staple fiber and at least 10% by weight of fibrid binding agent to [form a sheet, drying, and thereafter heat calendering the sheet to increase the bonding action of the fibrid binding agent upon the staple fibers; the improvement for providing a stronger and more uniform sheet without destructive zooalescense of the fibrous character of the fibrid binding agent and staple fibers wherein said calendering is conducted (a) upon said sheet While containing an initial amount of at least 10% by weight of water and (b) at a temperature at least 20 C. below the melting point of the lowest melting polymeric component in the sheet, and between about C. and about C., and at a nip pressure of at least 5 pounds per inch.

2. The process of claim 1 wherein the synthetic polymer forming the staple fiber is poly(hexamethylene adipamide).

3. The process of claim 1 wherein the synthetic polymer forming the staple fiber is polyethylene terephthalate.

4. The process of claim 1 wherein the synthetic polymer forming the fibrid binding agent is a oopolymer containing hexamethylene a'dipamide and caproamide units.

5. The process of claim 1 wherein the synthetic polymer forming the staple fiber is a copolymer containing ethylene terephthalate and ethylene isophthalate units.

References Cited in the file of this patent UNITED STATES PATENTS 2,999,788 Morgan Sept. 12, 1961 FOREIGN PATENTS 572,962 Great Britain Oct. 3 1, 1945 

1. IN THE PROCESS FOR FORMING A NON-WOVEN, FUSION BONDED, COMPOSITE SHEET FROM SYNTHETIC POLYMER STAPLE FIBER AND FLUID BINDING AGENT, SAID POLYMER BEING SELECTED FROM THE GROUP CONSISTING OF POLY(HEXAMETHYLENE ADIPAMIDE), POLYCAPROAMIDE, POLYETHYLENE TEREPHTLALATE, COPOLYMERS CONTAINING HEXAMETHYLENE ADIPAMIDE AND CAPROAMIDE UNITS, AND COPOLYMERS CONTAINING ETHYLENE TEREPHTHALATE AND ETHYLENE ISOPHTHALATE UNITS, BY CONCOMITANTLY DEPOSITING AN AQUEOUS SLURRY OF A MIXTURE OF SAID STAPLE FIBER AND AT LEAST 10% BY WEIGHT OF FIBRID BINDING AGENT TO FORM A SHEET, DRYING, AND THEREAFTER HEAT CALENDERING THE SHEET TO INCREASE THE BONDING ACTION OF THE FIBRID BINDING AGENT UPON THE STAPLE FIBERS; THE IMPROVEMENT FOR PROVIDING A STRONGER AND MORE UNIFORM SHEET WITHOUT DESTRUCTIVE COALESCENSE OF THE FIBROUS CHARACTER OF THE FIBRID BINDING AGENT AND STAPLE FIBERS WHEREIN SAID CALENDERING IS CONDUCTED (A) UPON SAID SHEET WHILE CONTAINING AN INITIAL AMOUNT OF AT LEAST 10% BY WEIGHT OF WATER AND (B) AT A TEMPERATURE AT LEAST 20* C. BELOW THE MELTING POINT OF THE LOWEST MELTING POLYMERIC COMPONENT IN THE SHEET, AND BETWEEN ABOUT 105* C. AND ABOUT 140*C., AND AT A NIP PRESSURE OF AT LEAST 45 POUNDS PER INCH. 